Method for producing a component by hot forming a pre-product made of steel

ABSTRACT

A method for producing a component by hot forming a pre-product made of steel is disclosed. The pre-product is heated to a forming temperature and is then reshaped, said component having a bainitic microstructure with a minimum tensile strength of 800 MPa after the forming process. In the process, the pre-product with the specified alloy composition is heated to a temperature below the A c1  transformation temperature, said pre-product already consisting of a steel with a microstructure made of at least 50% bainite.

The invention relates to a method for producing a component by hot forming a pre-product made of steel according to the preamble of patent claim 1. In the following, the term pre-product means for example sheets cut form coil or cut plates or seamless or welded tubes, which optionally may additionally be cold drawn.

Such components are mainly used in the automobile and utility vehicle industry, however they may also find use in machine constriction or building construction.

The hotly contested market forces automobile manufacturers to constantly seek solutions for lowering fleet consumption, while retaining a highest possible comfort and vehicle occupant protection. In this context an important factor is on one hand to save weight in all vehicle components, however, on the other hand also a favorable behavior of the individual components when subjected to high static and dynamic stress during operation or in the case of a crash.

Pre-material manufacturers seek to address this requirement by providing high-strength and ultra-high-strength steels, which allow reducing the wall thicknesses and at the same time possess improved component properties during manufacturing and during operation.

These steels therefore have to meet relatively high demands regarding strength, ductility, tenacity energy absorption and corrosion resistance and their processability for example during cold forming and during welding.

In light of the aforementioned aspects, manufacturing components from hot formable steels gains increasing importance because these ideally meet the heightened requirements at low material costs.

The production of components by means of quenching of pre-products made of press-hardenable steels by hot forming in a forming tool is known from DE 601 19 826 T2. In this case a sheet metal blank, which was heated beforehand above austenitization temperature to 800-1200° C. and is optionally provided with a coating of zinc or on zinc basis, is formed into a component in an optionally cooled tool by hot forming, wherein the sheet or component is quench hardened (press-hardening) in the forming tool during the forming by quickly withdrawing heat and thereby attains the demanded microstructure and strength properties.

The metallic coating is applied as corrosion protection, usually by means of the continuous hot dip coating method, onto a hot strip or cold strip or onto the pre-product produced therefrom, for example as hot galvanizing or hot dip aluminizing.

Subsequently the plate is cut to size for the forming tool used for the hot forming. It is also possible to provide the work piece or the blank to be formed with a hot dip coating.

The application of a metallic coating onto the pre-product to be formed prior to the hot forming is advantageous in this process because the coating effectively avoids scaling of the basic material and due to an additional lubricating effect avoids excessive tool wear.

Known hot-formable steels for this field of application are for example the manganese-boron steel “22MnB5” and recently also air hardenable steels according to DE 10 2010 024 664 A1.

In order to obtain components with very high strengths of more than 980 MPa, which still have a sufficient tenacity, it is known from EP 2 546 375 A1 to form a steel with a microstructure, which in the initial state is ferritic, by means of die-hardening and to establish a microstructure of bainite, tempered martensite and residual austenite in the finished component by a stepwise process. Hereby the sheet metal to be formed is first heated to a temperature of 750 to 1000° C. and is held at this temperature for 5 to 1000 seconds, subsequently formed at 350 to 900° C. and cooled to 50 to 300° C. Finally, the component is reheated to a temperature of 350 to 490° C., which is maintained for a time period from 5 to 1000 seconds. The microstructure of the finished component in this case is composed of 10 to 85% martensite, 5 to 40% residual austenite and at least 5% bainite.

However the production of a component by hot forming by means of press-hardening has several disadvantages.

On one hand, this method requires heating a pre-product to austenitization temperature and also requires great amounts of energy during the transformation of ferrite into austenite, which makes this process expensive and produces significant amounts of CO₂.

In addition, in order to avoid excessive scaling of the sheet metal surface an additional metallic protective layer or a protective layer on lacquer basis as described above or significant post processing of the scaled surface resulting from the heating and forming is required.

Because the forming at above Ac₃ temperature is usually performed at temperatures significantly above 800° C., extremely high demands are placed on the temperature stability of these layers.

A further disadvantage is also that for obtaining corresponding component strengths after the press-hardening, only transformation-capable steels with a sufficient transformation-inertness can be used, which correspondingly have to contain expensive alloy additions to achieve the desired microstructure and hardness after the forming.

In summary, the known method for producing components made of steel by hot forming above austenitization temperature leads to high manufacturing and energy costs due to the required large furnaces in combination with long incubation times.

For improving the forming capability of high-strength steels DE 10 2004 028 236 B3 discloses to further process work pieces to a component by hot forming at temperatures of 400 to 700° C. instead of cold forming (warm forming). A disadvantage hereby is that the formed component undergoes softening as a result of heating below the transformation temperature, i.e., the strength compared to the starting state is reduced.

DE 10 2011 108 162 A1 discloses a method for producing a component by warm forming a pre-product made of steel below AC₁-transformation temperature, in which a strength increase in the component is achieved by cold forming the pre-product prior to heating to forming temperature. Optionally an additional strength increase can be achieved in the component by using higher strength materials, such as bainitc, martensitic, micro-alloyed and dual-phase or multiphase steels. A disadvantage is hereby the additional costs due to the required cold forming prior to the heating to forming temperature. During hot forming dual-phase steels also have the disadvantage of being sensitive against edge brake induced failure during the forming.

Concrete alloy compositions to be adhered to or specifications regarding the microstructure of the pre-product for setting the mechanical properties of the component after warm forming in a targeted manner when using higher-strength steels are not disclosed.

It is an object of the invention to set forth a method for producing a component by hot forming a pre-product made of steel at temperatures below the Ac₁-transformation point, which is cost-effective and with which properties of the formed component can be achieved that are comparable to or improved relative to the known hot forming by press hardening, in particular the goal is to achieve strengths of more than 800 MPa at yield strength of more than 700 MPa and elongation at break A₈₀ of more than 8% of the finished component and a ductile failure behavior of the component.

According to the invention, this object is solved by a method for producing a component by hot forming a pre-product made of steel, in which the pre-product is heated to forming temperature and is subsequently formed, wherein after the forming the component has a bainitic microstructure with a minimal tensile strength of 800 MPa, which is characterized in that the pre-product is heated to a temperature below the Ac₁-transformation temperature, wherein the pre-product is made of a steel that already has a microstructure of at least 50% bainite, and wherein the pre-product has the following alloy composition in weight %:

-   C: 0.02 to 0.3 -   Si: 0.01 to 0.5 -   Mn: 1.0 to 3.0 -   P: max 0.02 -   S: max 0.01 -   N: max 0.01 -   Al: up to 0.1 -   Cu: up to 0.2 -   Cr: up to 3.0 -   Ni: up to 0.2 -   Mo: up to 0.2 -   Ti: up to 0.2 -   V: up to 0.2 -   Nb: up to 0.1 -   B: up to 0.01

Compared to methods known from DE 601 19 826 T2 or EP 2 546 375 A1 for producing a component by means of press hardening, the method according to the invention has the advantage that a component with mechanical characteristic values that are equal to or better than the mechanical properties of the pre-product in its initial state is provided at significantly lower energy requirement for the heating by using a steel which is already bainitic in its initial state. This saves energy costs.

Compared to DE 10 2011 108 162 A1, using the alloy composition according to the invention and a pre-product which in its initial state has already a microstructure with at least 50% bainite, the additional step of a cold forming of the pre-product for increasing strength is not required, and the demanded mechanical properties of the component can be adjusted in a targeted manner after the warm forming.

Using a steel for the pre-product which has the stated alloy composition and is already bainitic is very advantageous because already the starting material has a high tensile strength and ductility which are also retained or are even higher after the (transformation-free) forming.

The bainitic steel used for the method according to the invention is provided with its microstructure via a corresponding temperature profile already during production of the pre-product. In the case of hot strip, the microstructure can for example be established via thermo-mechanical rolling, in the case of cold strip for example by an annealing process after the cold rolling or also during the hot dip galvanizing.

A “softening” as it was observed in other high-strength steels after the forming could not be observed in this bainitic steel. A “softening” is oftentimes associated with a microstructure transformation and is thus time- and temperature critical. The use of the pre-product according to the invention made of metallic bainitic steel on the other hand is largely insensitive, so that for example intended and unintended time and temperature variations during heating and forming do not result in an impairment of the mechanical properties. As a result of this advantageous material behavior also sophisticated multi-stage process steps can be reproducibly performed.

The particular advantage of using this alloy concept and the bainitic microstructure is also a very fine and homogenous microstructure with at least 50% bainite and only small proportions of ferrite, residual austenite and martensite.

Especially advantageous for achieving the demanded mechanical properties is when the microstructure has at least 70% bainite and the proportions of residual austenite+martensite are<10% and the remainder consists of iron.

Particularly uniform and homogenous material properties can be achieved when the bainitic steel of the pre-product has the following alloy composition in weight %:

-   C: 0.02 to 0.11% -   Si: 0.01 to 0.5% -   Mn: 1.0 to 2.0% -   P: max 0.02% -   S: max 0.01% -   N: max 0.01% -   Al_(min): 0.015 to 0.1% -   B: max. 0.004% -   Nb V+Ti: max 0.2%

In a further improved embodiment of the invention the steel of the pre-product has the following alloy composition in weight %:

-   C: 0.05 to 0.11% -   Si: 0.01 to 0.5% -   Mn: 1.0 to 2.0% -   P: max. 0.02% -   S: max 0.01% -   N: 0.003 to 0.01% -   Al_(min): 0.03 to 0.1% -   B: max 0.004% -   Mo: 0.04 to 0.2 -   Ti: 0.04 to 0.2 -   Nb+V+Ti: 0.1 top 0.2%

The addition of nitrogen of at least 0.03 to 0.01 weight % ensures in combination with carbon and a minimal content of titanium of 0.04 to 0.2 weight % a fine grained microstructure as a result of the formation of titanium carbonitrides with high strength and tenacity properties. As a result of the addition of molybdenum at contents of 0I04 to 0.2 weight % the forming precipitations are also advantageously kept very small.

Comparative tests were performed on steels with the alloy compositions stated in table 12. The results for the mechanical properties prior to and after the warm forming are shown in table 2.

The sheet metals that were tested had a thickness of 1.8 to 2.25 mm, which were heated in the furnace to a temperature of 600° C. for 3 minutes and subsequently cooled between two even tool components in the forming press.

The tested materials are indicated in tables 1. and 2 with the letters a, b, c, d, e and f. The alloy compositions of the materials correspond to that according to the invention, wherein the microstructure, however, were set to be different in the starting state. Thus in the initial state, steel a had a ferritic-bainitic basic microstructure “FB” prior to the heating to forming temperature, steel b had a bainitic “B” microstructure, steel c had a mixed microstructure of martensite, bainite and ferrite “MBF”, wherein the martensite content dominates. The steels d and e had a ferritic “F” microstructure and steel f had a martensitic microstructure “M”. In the steels a and c, the bainite content in the microstructure was below 50% and in steel b above 50%. Table 2 shows that only steel b with a predominant bainitic starting microstructure of the pre-product satisfies the requirements placed on the mechanical properties with a minimal tensile strength of 800 MPa and a minimal elongation at break A₈₀ of more than 8% after the warm forming.

Typical applications for utilizing the high strength potential at simultaneous weight saving on the component are the mobile derrick construction, longitudinal members and transverse members in trucks and trailers, safety parts and chassis parts in automobiles and train car construction.

The steel according to the invention or the component produced therefrom is characterized by a very high yield strength and tensile strength of more than 800 MPa at sufficient ductility. In addition, the chemical composition also results in a good weldability.

The above-mentioned steel, as is known, can further be provided with a scale resistant or corrosion-resistant layer on lacquer basis or with a metallic coating. The metallic coating can contain zinc and/or magnesium and/or aluminum and/or silicone.

In contrast to the conventional production routes an already surface treated hot strip or cold strip can be used for the forming subsequent to a heating, because the adhesion and ductility withstands a warm forming with low degrees of deformation. The metallic coating is resistant against short-time reheating of the combination substrate/coating (steel strip/coating) below Ac₁ temperature of the substrate, in order to withstand the reheating prior to the warm forming and the actual warm forming.

Due to the relatively small heat amount, large reheating aggregates such as pusher-type furnaces or batch-type furnaces, can be dispensed with in favor of fast and direct-acting systems (inductive, conductive and in particular radiation).

In addition the described method also requires significantly less heat energy, i.e., the energetic efficiency is higher than in press-hardening. As a result the process costs are lower and CO₂ emission is reduced.

Preferably the reheating occurs prior to the warm forming by means of radiation, because in this case the efficiency is significantly higher than in case of heating in a furnace or in case of conductive heating and the energy input into the material is faster and more effective depending on the surface conditions.

The material is suited very well for a partial heating. By using for example radiators, individual regions of the pre-product to be formed can be heated in a targeted manner in order to obtain zones that are optimized regarding their forming capability. This makes it possible to advantageously use conventional dies for cold forming so that a complex hot forming system as required in press-hardening is not required.

For transport between the heat source and the forming tool it can further be useful, in particular in the case of very thin sheet metals (for example<0.8 mm), to provide the cut sheet metals with a profiling for increasing the local stiffness. This is not possible in conventional press-hardening because the strength to be achieved requires an abrupt cooling via the inner face of the tool, which is excluded due to the profiling.

In the method according to the invention the pre-product is heated to a temperature below 720° C., advantageously in a temperature range of 400-700° C. and is subsequently formed to a component. The optimal forming temperature depends on the demanded strength of the component and is preferably between 500° C. and 700° C. Long holding times, in order to obtain a bainitic microstructure such as described in EP 2 546 375 A1 are not required so that the processing time for the production of the component is significantly shortened.

In an advantageous embodiment of the invention a local exceeding of the temperature range of the warm forming in the austenite region is performed during heating of the pre-product to forming temperature, in order to change properties locally in a targeted manner (for example local hardening) which in combination with the strength increase of the remaining material is adjusted to the later demands on the component.

TABLE 1 Material C SI Mn P S N Al Cu Cr Ni V Ti Nb Mo B FB a 0.07 0.08 1.4 0.01 0.002 0.005 0.041 0.03 0.04 0.04 0.05 — 0.04 — — B b 0.08 0.47 1.9 0.01 0.001 0.006 0.066 0.03 0.03 0.04 0.01 0.12 0.05 0.14 — MBF c 0.23 0.25 1.2 0.01 0.002 0.005 0.038 0.04 0.16 0.04 0.01 0.03 — — 0.003 F d 0.10 0.28 2.0 0.01 0.001 0.006 0.041 0.02 0.33 0.04 0.01 0.04 0.04 — 0.003 M f 0.15 0.12 1.7 0.01 0.001 0.005 0.045 0.02 0.33 0.04 0.01 0.02 — — — F e 0.09 0.25 1.8 0.01 0.001 0.005 0.041 0.03 0.33 0.04 0.01 — 0.01 — —

TABLE 2 Tensile strength R_(m) [MPa] Yield strength [MPa] Elongation at break A₈₀ [%] Material Prior to HWU After HWU Prior to HWU After HWU Prior to HWU After to HWU FB a 591 632 552 589 19 16 B b 788 854 678 833 14 12 MBF c 982 979 915 922 7 7 F d 855 778 644 767 13 12 M f 1343 1246 1047 1173 6 1 F e 676 650 407 498 22 18 

What is claimed: 1.-15. (canceled)
 16. A method for producing a component by hot forming a pre-product made of steel, comprising: providing a pre-product made of steel having a microstructure of at least 50% bainite and having the following alloy composition in weight %: C: 0.02 to 0.3 Si: 0.01 to 0.5 Mn: 1.0 to 3.0 P: max 0.02 S: max 0.01 N: max 0.01 Al: up to 0.1 Cu: up to 0.2 Cr: up to 3.0 Ni: up to 0.2 Mo: up to 0.2 Ti: up to 0.2 V: up to 0.2 Nb: up to 0.1 B: up to 0.01; heating the pre-product to forming temperature below Ac1 transformation temperature; and forming the pre-product into the component, wherein the component after the forming has a bainitic microstructure with a minimal tensile strength of 800 MPa.
 17. The method of claim 16, wherein the microstructure of the pr-product is composed of at least 70% bainite and a content of residual austenite+martensite is<10% and the remainder is ferrite.
 18. The method of claim 16, wherein the pre-product has the following alloy composition in weight %: C: 0.02 to 0.11% Si: 0.01 to 0.5% Mn: 1.0 to 2.0% P: max 0.02% S: max 0.01% N: max 0.01% Al_(min): 0.015 to 0.1% B: max. 0.004% Nb+V+Ti: max 0.2%.
 19. The method of claim 16, wherein the pre-product has the following alloy composition in weight %: C: 0.05 to 0.11% Si: 0.01 to 0.5% Mn: 1.0 to 2.0% P: max. 0.02% S: max 0.01% N: 0.003 to 0.01% Al_(min): 0.03 to 0.1% B: max 0.004% Mo: 0.04 to 0.2 Ti: 0.04 to 0.2 Nb+V+Ti: 0.1 to 0.2%.
 20. The method of claim 16, wherein during the heating step only portions of the pre-product are heated to forming temperature, and optionally above the Ac₁ transformation temperature.
 21. The method of claim 16, wherein the pre-product is heated to a temperature below 720° C.
 22. The method of claim 21, wherein the pre-product is heated to a temperature in a ranged from 400 to 720° C.
 23. The method of claim 22, wherein the pre-product is heated to a temperature in a ranged from 500 to 700° C.
 24. The method of claim 16, further comprising prior to the heating step, is providing the pre-product with a metallic or lacquer-like coating.
 25. The method of claim 24, wherein the metallic coating contains Zn and/or Mn and/or Al and/or Si.
 26. The method of claim 16, wherein the heating to forming temperature is accomplished inductively, conductively, or by means of radiation.
 27. The method of claim 16, wherein the pre-product is a metal plate or a tube.
 28. The method of claim 27, wherein the metal plate is made of hot strip or cold strip.
 29. The method of claim 27, wherein the tube is a seamlessly hot rolled tube or a welded tube made of hot strip or cold strip.
 30. The method of claim 29, wherein the tube is subjected to one or multiple further drawing and/or annealing processes. 